DPPH Radical Scavenging Activity of Tricin and Its Conjugates Isolated from Njavara Rice Bran: A Density Functional Theory Study

Structural, electronic, and energetic characteristics of tricin, tricin-4'-O-(erythro-beta-guaiacylglyceryl)ether (TEGE), and tricin-4'-O-(threo-beta-guaiacylglyceryl)ether (TTGE), isolated from Njavara rice bran have been studied using DFT to explain their experimentally determined radical scavenging activity (EC50 values) in comparison with known standards such as quercetin, myricetin, and catechin. Among the three mechanisms proposed for explaining the antioxidant activity, proton coupled-electron transfer (PC-ET), sequential proton loss electron transfer (SPLET), and electron transfer-proton transfer (ET-PT), our results support the second one. The O-H bond dissociation enthalpy (BDE) and the spin density on the oxygen with the radical character are excellent descriptors of radical scavenging activity. BDE (in kcal/mol) increased in the order myricetin (74.6) < quercetin < catechin (78.3) < tricin (81.5) < TTGE (90.6) < TEGE (91.1), while the EC50 increased exponentially with increase in BDE, 20.51, 42.98, 45.07, 90.39, 208.01, and 352.04 mu g/mL for myricetin, quercetin, catechin, tricin, TTGE, and TEGE, respectively.